There can be more causes for increased platelets, among which there can be a primary increase, which is seen in myeloproliferative disorders such as true erythrocytosis and primary thrombocytosis. In chronic granulocytic leukemia or in the early stages of myelofibrosis, thrombocytosis can also occur, and this number is significantly higher than 400 g/L early on, and this lasts longer. It is likely that this is not the case when platelets are elevated during fever, because a reactive increase in platelets can also occur during fever, which is often a case of acute infection, and after the fever is over, the platelets are able to fully return to the normal range. These myeloproliferative disorders are only considered if the platelet count does not return to normal after the fever has persisted for some time, or if the fever is completely over and the platelet count is still elevated instead.